In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see Non Patent Document 1).
In the D2D communication, a plurality of adjacent user terminals perform direct device-to-device communication without passing through a core network. On the other hand, a data path of normal communication (cellular communication) of a mobile communication system passes through the core network.
In the D2D communication, it is supposed that an uplink radio resource or a downlink radio resource of cellular communication is used. The uplink radio resource includes a plurality of uplink subframes divided on a time axis. The downlink radio resource includes a plurality of downlink subframes divided on a time axis.